1. Field of the Invention
The present invention relates to a control device that detects whether or not irreversible demagnetization has occurred in a permanent magnet of a permanent magnet synchronous motor, which operates a driven object, such as a work and tool, in a machine tool, etc.
2. Description of Related Art
Conventionally, a permanent magnet synchronous motor, such as a rotary servomotor in which a permanent magnet is provided in any one of a stator and a rotor, a liner servomotor in which a permanent magnet is provided in any one of a stator and a slider, and a vibration servomotor in which a permanent magnet is provided in any one of a stator and a vibrator, is widely used in various fields.
In a permanent magnet synchronous motor, there is a case where irreversible demagnetization occurs in a permanent magnet of the permanent magnet synchronous motor due to an abnormal temperature rise of the permanent magnet of the permanent magnet synchronous motor, an abnormal drive current that flows through the winding of the permanent magnet synchronous motor, etc.
In such a case, it is not possible for the permanent magnet synchronous motor to generate a predetermined torque, and therefore, the actual track of movement of a driven object, that is operated by the permanent magnet synchronous motor, deviates from the track of movement of the driven object commanded by an upper control device, such as a CNC (computer numerical control), and there is a possibility that the track precision deteriorates and the driven object is broken under certain circumstances.
Consequently, it is necessary to detect whether or not irreversible demagnetization has occurred in the permanent magnet of the permanent magnet synchronous motor in order to avoid the deterioration of the track precision and the breakage of the driven object under certain circumstances due to the reduction of acceleration and deceleration performance caused by the irreversible demagnetization in the permanent magnet of the permanent magnet synchronous motor.
Conventionally, a control device that specifies the position of the permanent magnet in which irreversible demagnetization has occurred by comparing the calculated inertia of the driven object of the permanent magnet synchronous motor and a predetermined inertia of the permanent magnet synchronous motor in each magnetic pole position detected based on a rotation angle of a rotor of the permanent magnet synchronous motor in order to detect irreversible demagnetization in the permanent magnet of the permanent magnet synchronous motor, is proposed in, for example, Japanese Patent No. 3857425 (JP3857425B2).
However, the conventional control device compares the calculated inertia and the predetermined inertia in each position of the permanent magnet, and therefore, it is necessary to rotate the rotor of the permanent magnet synchronous motor at least one electrical angle or more. Due to this, when the operation range of the drive shaft of the permanent magnet synchronous motor is limited, such as when the driven object is moved along an arc using the permanent magnet synchronous motor in a machine tool etc., and therefore, it is not possible to rotate the rotor of the permanent magnet synchronous motor one electrical angle or more, it is not possible to specify the position of irreversible demagnetization in the permanent magnet of the permanent magnet synchronous motor. Further, the conventional control device detects demagnetization indirectly by estimating the inertia. In this case, calculation is performed by dividing the product of the torque constant and the detected current by the detected acceleration, however, in general, the acceleration is calculated from a difference in speed, and therefore, noise is likely to be included and an error due to division is likely to be included. Therefore, when the acceleration is small, there is a case where the operation precision deteriorates considerably.